A Gemini North view of 15 Sge (left) with the image processed (right) to show the brown dwarf more clearly

By BBC News Online science editor Dr David Whitehouse

Astronomers have caught a remarkable image of a brown dwarf circling a nearby star.

A brown dwarf is too big to be a planet but too small to be a star and although a great many have been detected before, this is the first time one has been directly imaged so close to its companion.

It was done using the relatively new technique of adaptive optics, which allows astronomers to get a much clearer view through the Earth's turbulent and distorting atmosphere.

The discovery raises questions about how brown dwarfs and planets are formed.

Failed star

Because brown dwarfs are intermediate objects between planets and stars, they are often described as "failed stars"; they are more massive than Jupiter, the largest planet in our Solar System, but fall short of the minimum mass needed to sustain nuclear fusion. This is estimated to be 8% of the Sun's mass.

A Keck Observatory image of 15 Sge with the brown dwarf arrowed

The brown dwarf is orbiting a star called 15 Sagittae (Sge), which is about one to three billion years old, making it slightly younger than our Sun. It is located approximately 58 light-years from Earth.

It is separated from its parent star by less than the distance that separates the planet Uranus from the Sun. This makes it the smallest-separation brown-dwarf companion yet seen directly. The research team behind the discovery estimate the mass of the brown dwarf to be 55 to 78 times the mass of planet Jupiter.

It is an observation that raises questions about how the brown dwarf formed. It also adds to the surprising diversity of extrasolar planetary systems now being found.

Twinkle effect

"This discovery implies that brown-dwarf companions to average, Sun-like stars exist at a separation comparable with the distance between the Sun and the outer planets in our Solar System," said Michael Liu, of the University of Hawaii's Institute for Astronomy, US.

"This companion is probably too massive to have formed the way we believe that planets do, namely from a circumstellar disc of gas and dust when the star was young," Liu said.

Hints of an interesting object around 15 Sge first arose in data gathered 10 years ago from Lick Observatory in California. More recently, astronomers looked for planets circling the star. While they did not find any planets, they did notice clues of a more massive, distant companion.

The brown dwarf was found using a relatively new observing technique called adaptive optics. The system was used on two telescopes in Hawaii. It utilises a flexible mirror that is constantly being bent slightly to compensate for the distortion of the atmosphere - the same effect that makes the stars twinkle when we look up at the night sky.

Future feast

Last summer, Liu and colleagues took high-resolution pictures of 15 Sge using the adaptive optics (AO) system on the Gemini North telescope. Further observations were obtained using the Keck AO system in August and December 2001.

"Only by using adaptive optics to produce very sharp images could we have found this companion," Liu said. "It is too faint and too close to its parent star to be seen otherwise."

Other astronomers are full of praise for the finding. "I think these discoveries are both marvellous, but in many ways they're really just a very tantalising appetiser for what's to come," said Alan Boss, of the Carnegie Institute of Washington, US.

"In the next few years, we're likely to find... a feast of results from adaptive optics systems that function very well on large telescopes," he added.